Reaction for high performance (Bi,Pb)2 Sr2 Ca2 Cu3 Oy composites

ABSTRACT

The present invention provides a (Bi,Pb)SCCO-2223 oxide superconductor composite which exhibits improved critical current density and critical current density retention in the presence of magnetic fields. Retention of critical current density in 0.1 T fields (77 K, ⊥ ab plane) of greater than 35% is disclosed. Significant improvements in oxide superconductor wire current carrying capacity in a magnetic field are obtained by subjecting the oxide superconductor composite to a post-processing heat treatment which reduces the amount of lead in the (Bi,Pb)SCCO-2223 phase and forms a lead-rich non-superconducting phase. The heat treatment is carried out under conditions which localize the lead-rich phase at high energy sites in the composite.

FIELD OF THE INVENTION

The invention relates to high performance oxide superconductorcomposites exhibiting improved J_(c) retention in the presence of amagnetic field. The invention further relates to a method forpost-formation processing of an oxide superconductor composite toimprove electrical performance.

BACKGROUND OF THE INVENTION

In order to obtain high electrical performance in (Bi,Pb)₂ Sr₂ Ca₂ Cu₃O_(y) ((Bi,Pb)SCCO-2223) high temperature superconducting composites,highly phase pure (Bi,Pb)SCCO-2223 with perfect texture and superiorgrain connectivity is desired. "Texture" refers to the degree ofalignment of the oxide superconductor grains along the direction ofcurrent flow. "Connectivity" refers to the positional relationship ofthe oriented oxide superconductor grains, the nature of the grainsboundaries and the presence of phase impurities disrupting intergrainconnection.

Many parameters must be controlled and optimized during the fabricationand thermomechanical processing of (Bi,Pb)SCCO-2223 tapes in order toobtain satisfactory electrical properties. Electrical properties may begrouped into two categories: intragranular electrical properties andintergranular electrical properties. Intragranular electrical propertiesare those that are effected by changes within individual oxidesuperconductor grains. Critical transition temperatures (T_(c)) is oneelectrical property which is predominantly intragrain. Critical currentdensity (J_(c)) and critical current retention (J_(ret)) also has anintragrain component. Intergranular electrical properties are thosewhich relate to the transport of a supercurrent across oxidesuperconducting grain boundaries and depend upon good intergrainconnectivity. Critical current density (J_(c)) and critical currentretention in a magnetic field (J_(ret)) have significant intergraincharacter.

In references too numerous to identify individually, the effects ofpowder composition, mechanical deformation, and heat treatment time,temperature and atmosphere on oxide superconductor formation have beenstudied. Not surprisingly, these studies have shown that heat treatmentaffects the rate of formation of the superconductor phase, the qualityof the superconductor phase and the presence of secondary,non-superconducting phases. Thus, the heat treatment used in theformation of the oxide superconductor phase is important to the overallperformance of the oxide superconductor composite.

Post-formation heat treatments have been investigated as a means formodifying the intragranular structure to boost performance properties ofthe oxide superconductor phase. Intragrain factors which affectelectrical properties include the presence or absence of defects in thesuperconductor phase, and the phase purity of the superconductor phaseand stoichiometric modifications thereof which may improve or degradesuperconducting behavior. "Post-formation", as that term is used herein,means processing of the oxide superconductor after formation of thedesired oxide superconductor phase from precursor oxide phases issubstantially complete.

Typical post-processing heat treatments include annealing to alter theoxygen stoichiometry of the oxide superconductor phase, such asdescribed by E. Ozdas and T. Firat in "Oxygenation Intercalation andIntergranular Coupling in the 110-K Bi₁.7 Pb₀.3 Sr₁.8 Ca₂ Cu₂.8 O₉.45+δSuperconductor" (Phys. Rev. B 48(13):9754-9762 (October 1993)) andIdemoto et al. in "Oxygen Nonstoichiometry of 2223 PhaseBi--Pb--Sr--Ca--Cu--O System Superconducting Oxide" (Physica C181:171-178 (1991)). They reported on the effect of heating(Bi,Pb)SCCO-2223 powders at temperatures from 500° C. to 800° C. andoxygen pressures of 0.2 to 10⁻³ atm. Idemoto et al. observed theformation of secondary phase Ca₂ PbO₄ and evaporation of PbO, whileOzdas and Firat reported inhomogeneities forming at oxide superconductorgrain boundaries.

Um et al. (Jpn. J. Appl. Phys. 32: 3799-3803 (1993)) investigated theeffect of a post-sintering anneal on (Bi,Pb)SCCO-2223 powders. Theyobserve that T_(c) is affected by the anneal temperature and oxygenpressure and found annealing at temperatures below 700° C. and at oxygenpartial pressure of 0.01 atm to provide optimized T_(c). Um et al. notedthat the superconducting phase decomposes at temperatures higher than700° C. Wang et al. (Advances in Supercond. V (1992)) also found thatpost-annealing under vacuum at 790° C. improved T_(c) of(Bi,Pb)SCCO-2223 oxide superconductor powders.

These prior art references investigate the intragranular electricalproperties of oxide superconductor powders and the authors are primarilyinterested in T_(c) optimization. Oxide powders have no intergranularboundaries because of the random loose-packed nature of powder, and theyprovide no insight into the optimization of electrical transportproperties (J_(c), J_(ret)) of (Bi,Pb)SCCO-2223 superconductor currentcarriers, such as wires, tapes and the like.

Interestingly, the above-mentioned prior art noted the decomposition ofthe superconducting oxide phase and formation of secondary phases whileoptimizing intragranular electrical properties. Conventional wisdomwould suggest that microstructures containing a non-superconductingsecondary are undesirable because these particles disrupt localalignment of the BSCCO-2223 grains and decrease superconducting volumefraction in the composite. Thus, prior investigations have suggestedthat it is highly desirable to reduce the amount of secondary phases toas low a level as possible.

There has been little or no investigation of conditions which optimizethe interconnectivity of (Bi,Pb)SCCO-2223 superconductor grains in asilver sheathed wire or which investigate its retention of criticalcurrent in the presence of a magnetic field. In the case of silversheathed high temperature superconducting wires, good intergranularconnectivity is critical to performance, yet processing is complicatedby the need to move oxygen through the silver to the oxidesuperconductor. Observations made for oxide superconductor powders,which are an open system exposed directly to the furnace atmosphere andwhich systems do not include silver/oxygen interfaces, may not apply tosilver sheathed tapes and the like.

The effects of cooling on the electrical properties of the oxidesuperconductor composite has been investigated by Lay et al. in"Post-Sintering Oxygen Pressure Effects on the Jc of BPSCC-Silver CladTapes" (Mat. Res. Symp. Proc. 275:651-661 (October, 1992). Lay et al.reported cooling in air at 1° C./min resulted in a J_(c) (77 K, 0 T)increase over tapes cooled at 3° C./min. Lay et al. also noted thatholding the (Bi,Pb)SCCO-2223 samples at temperatures of 810° C. or 780°C. in reducing atmospheres improved J_(c).

While critical current (I_(c)) and critical current density (J_(c)) inself-fields may be useful indications of the quality of an oxidesuperconductor composite, an important performance parameter forin-field operations of oxide superconducting devices is their ability toretain their superconducting transport properties in the presence of amagnetic field. Many applications using oxide superconducting wires mustbe accomplished in the presence of its own induced magnetic field or inapplied field ranging from 0.01 T to 100 T. Superconducting propertiesdegrade dramatically in even relatively weak fields. Oxidesuperconductors show their most dramatic loss in critical currentcapacity perpendicular to the ab plane. Parallel to the ab plane,capacity loss is only a few percent. For example, weakly linkedyttrium-barium-copper oxide superconductor (YBCO) exhibits a ten-folddrop in J_(c) in magnetic field strengths of 0.01 T (B ⊥ oxidesuperconductor tape plane). Conventionally processed BSCCO-2223 losesthe majority of its critical current capacity in a 0.1 T field (77 K, ⊥tape plane). Even a few percent increase in critical current retentionwould have a dramatic effect on wire performance.

Thus, there remains a need to optimize the intergrain connectivity ofhigh temperature superconducting wires and tapes so as to improvecurrent carrying performance. Preferably, processing of an oxidesuperconductor wire or tape would enhance the intragranular propertiesof the conductor without detriment to the intergranular transportproperties of the conductor. Due to secondary phase formation underconditions which optimize intragranular electrical properties, it isdesirable to process the superconductor in a manner which minimizes theformation and/or detrimental effect of secondary phases on intergrainconnectivity.

It is the object of the present invention to provide an oxidesuperconductor article with improved critical current retention and/orimproved critical current density in the presence of a magnetic field.

It is a further object of the present invention to provide a method oftreating the oxide superconductor composition to improve criticalcurrent retention and/or critical current density.

It is yet a further object of the present invention to increase fluxpinning sites and/or intragranular coupling in a (Bi,Pb)SCCO-2223 oxidesuperconductor composite.

It is yet a further object of the invention to improve graininterconnectivity by reducing formation and/or the detrimental effect ofsecondary phase formation.

It is yet a further object of the present invention to provide a methodform obtaining optimal intragranular properties of an oxidesuperconductor wire or tape, while minimizing the detrimental effects tointergranular transport properties.

These and other objection of the invention are accomplished by theinvention as set forth hereinbelow.

SUMMARY OF THE INVENTION

The present invention provides a (Bi,Pb)SCCO-2223 oxide superconductorcomposite which exhibits improved critical current density (J_(e) orJ_(c)) and improved critical current retention (J_(ret)) the presence ofmagnetic fields. Retention of critical current density in 0.1 T fields(77 K, ⊥ to the tape plane) of up to about 40% have been observed; andcritical current retention of greater than about 30% is typical. Theimproved critical current retention is accompanied by the localizationof a lead-rich secondary phase at high energy sites within thecomposite. The present invention recognizes that, contrary to thecommonly held belief that secondary non-superconducting phases aredetrimental to superconducting electrical properties, enhanced criticalcurrent and critical current retention are obtained from a compositecontaining a lead-rich non-superconducting secondary phase.

In one aspect of the invention, a (Bi,Pb)SCCO-2223 oxide superconductorcomposite wire is provided having a (Bi,Pb)SCCO-2223 oxidesuperconductor filament substantially supported in a noble metal phase.The filament comprises a lead-rich secondary phase and the wire possessa J_(ret) at 0.1 T in the range of greater than 35% (77 K, ⊥ ab plane)when tested over a current carrying distance of 10 cm. The lead-richsecondary phase may be localized at high energy sites. The(Bi,Pb)SCCO-2223 may be lead deficient. In preferred embodiments, the(Bi,Pb)SCCO-2223 oxide superconductor phase comprises Bi:Pb:Sr:Ca:Cu inthe nominal stoichiometry of2.5(±0.05):0.4(±0.04):2.3(±0.06):2.3(±0.04):3.0(±0.15). Unless otherwisenoted, all references are to atomic percent. The composite mayadditionally include a lead-rich secondary phase comprisingBi:Pb:Sr:Ca:Cu in the nominal stoichiometry of0.9(±0.09):1.1(±0.21):1.6(±0.06):1.7(±0.08):1.0(±0.23).

The present invention further contemplates a (Bi,M)SCCO-2223 oxidesuperconductor wire including a (Bi,M)SCCO-2223 oxide superconductorfilament supported in a noble metal phase. M is may include Pb, Tl, Sb,Sn, Te, Hg, Se, As and mixtures thereof. The wire characterized in thatwhen tested over a current carrying distance of 10 cm, the wire possessa J_(ret) at 0.1 T of greater than 35% (77 K, ⊥ ab plane).

Significant improvements in oxide superconductor wire current carryingcapacity in a magnetic field are obtained by subjecting the oxidesuperconductor wire containing (Bi,Pb)SCCO-2223 to a post-processingheat treatment which reduces the lead content in the (Bi,Pb)SCCO-2223phase by an amount in the range of about 5 wt % to about 50 wt %, andtypically to about 40 wt %, and to localize the exsolved lead in alead-rich secondary phase outside the superconducting grain coloniesand/or at other high energy sites in the composite. Reduction of lead inthe (Bi,Pb)SCCO-2223 phase improves intragranular electrical properties.When the heat treatment is conducted under conditions which localizesecondary phases formed thereby at high energy sites, the secondaryphases do not significantly degrade the intergranular transportproperties of the composite.

The invention calls for the modification of the lead content of the a(Bi,Pb)SCCO-2223 superconducting phase during processing of a(Bi,Pb)SCCO-2223 oxide superconductor composite. The lead content variessuch that the lead content of the (Bi,Pb)SCCO-2223 superconducting phaseis in the range of 3% to 8%, preferably 6.5 wt %, during formation ofthe (Bi,Pb)SCCO-2223 phase and such that the lead content of the(Bi,Pb)SCCO-2223 superconducting phase is reduced up to 50% during postformation processing of the oxide superconductor phase. This results inthe optimization of the product superconductor electrical properties.

The method of the invention also contemplates heating the oxidesuperconductor composite under oxidizing conditions, said conditionssufficient to oxidize a portion of Pb²⁺ present in (Bi,Pb)SCCO-2223 intoPb⁴⁺ and to localize the Pb⁴⁺ in a secondary phase at high energy sitesof the composite.

The term "wire" is used herein to mean any of a variety of geometrieshaving an elongated dimension suitable for carrying current, such as butnot limited to wires, tapes, strips and rods.

By "fully formed (Bi,Pb)SCCO-2223", "desired (Bi,Pb)SCCO-2223", and"final (Bi,Pb)SCCO-2223" as those terms are used herein, it is meant anoxide superconductor phase in which substantially all of the precursoroxide has been converted into the desired (Bi,Pb)SCCO-2223 phase. Thereis no further processing into a different oxide superconductor phase.The (Bi,Pb)SCCO-2223 may be obtained according to the methods describedherein or according to other prior art methods demonstrated to completeconversion of the precursor oxides to (Bi,Pb)SCCO-2223.

"Critical current density retention", J_(ret), as that term is usedherein means the ratio of the critical current density of the compositein an applied field over the critical current density of the compositein the absence of an applied field (self field or zero field). Thesample will generate its own self field, but that field is expected tobe at least an order of magnitude less than the applied field.

High energy sites include high angle c-axis tilt boundaries, pores,interfaces between the superconducting and secondary phases and edgeboundaries for the superconducting phase. Oxide grains having amisorientation of greater than 10° angular deviation from perfectalignment of adjacent grains have a large relative proportion of highenergy sites.

The present invention is readily scalable and can be used to processlong lengths of oxide superconductor wire, in contrast to techniquessuch as irradiation, which are expected to introduce flux pinning andother site defects into the material.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described with reference to the Drawing, which ispresented for the purpose of illustration only and is no way intended tobe limiting of the invention, and in which:

FIG. 1 are X-diffraction patterns of (a) a hexagonal lead-rich secondaryphase and (b) a (Bi,Pb)SCCO-2223 composite of the invention includingthe hexagonal lead-rich secondary phase;

FIG. 2 is a plot of log P_(O2) (atm) vs. 1000/T (K) showing a lead-richphase reaction curve;

FIG. 3 is a temperature profile of the heat treatment of the inventioncarried out (a) in a single step; (b) as a series of steps; and (c) as aslow cooling step;

FIG. 4 is a heat treatment profile useful in preparing a(Bi,Pb)SCCO-2223 oxide superconductor;

FIG. 5 is a plot of critical current density as a function oftemperature (500° C. to 800° C.) in the post-formation heat treatment;

FIG. 6 heat treatment dwell time for oxide superconductor wires heattreated at 724° C. in 7.5% (0.075 atm) O₂ ;

FIG. 7 is a plot of critical current density as a function of oxygenpartial pressure (0.003-1.0 atm);

FIG. 8 is a bar graph illustrating critical current retention at 0.1 T(77 K, ⊥ tape plane) for a variety of post-formation heat treatments;

FIG. 9 is a plot of critical current retention vs. field strength for(Bi,Pb)SCCO-2223 wires post-formation heated under various oxygenpartial pressures;

FIG. 10 is a plot of critical current retention vs. field strength for(Bi,Pb)SCCO-2223 wires post-formation heated under various temperatures;

FIG. 11 is a plot of relative fraction lead-rich secondary phase as afunction of oxygen partial pressure;

FIG. 12 is a plot of relative fraction lead-rich secondary phase as afunction of temperature; and

FIG. 13 is a plot of J_(ret) v. B demonstrating the effect of heattreatment on critical current density retention above and below thelead-rich phase stability line.

DETAILED DESCRIPTION OF THE INVENTION

The critical current retention of BSCCO-2223 in a magnetic field isrelatively poor at high temperatures, e.g., 77 K. For example,conventionally processed BSCCO-2223 loses the majority of its criticalcurrent capacity in a 0.1 T field (77 K, ⊥ tape plane). Critical currentretention may be improved in two ways. In one method, defects can beintroduced into the superconductor phase that directly interact withmagnetic flux vortexes and impede their motion, so-called flux pinning.Defects may be particles giving rise to point, line, plane or volumedefects (zero, one, two or three dimensional defects) or particles whichcreate coherency strain fields or differential coefficients of thermalexpansion. In another method, the crystal lattice of the oxidesuperconductor itself is modified to improve the coupling of thecarriers responsible for superconducting behavior, so-called intrinsiccoupling. For example, carrier density may be modified by changing theoxide superconductor stoichiometry. Note that both of these mechanismsare intragranular.

In the case of high temperature superconducting wires, in whichmultifilaments of oxide superconductor are sheathed in a silver sheath,good intergranular connectivity is important to maintain effectivecurrent carrying capacity along the wire length. Good intergranularconnectivity must be maintained, even as the oxide superconductor wireis subjected to processes which enhance intragranular properties.

The prior art discussed hereinabove enhances intragranular properties(such as T_(c)) with a resultant decomposition of the oxidesuperconductor phase and formation of a secondary phase. Thus, previousefforts to enhance intragranular superconducting properties haveindicated that intergranular connectivity is substantially degraded asintragranular superconducting properties are enhanced.

Thus, some prior art investigations into post-sintering conditions havelead investigators to recommend regimes where formation of secondaryphases is minimized and intragranular properties such as T_(c) areoptimized. This may, however, be a processing regime which is not wellsuited to optimization of other electrical properties, in particular,critical current retention. For example, Um recommends post-sintering attemperatures of 500°-700° C. at partial oxygen pressures of 0.01 atm. Asis described herein, this processing regime may enhance T_(c) and avoidsecondary phase formation, but it does not enhance critical currentretention.

The present invention has recognized for the first time that processingconditions which optimize intragranular superconducting properties suchas T_(c) or critical current retention in a magnetic field (J_(ret) aredifferent than those processing conditions which optimize J_(c) at selffield or zero field, a property having a dominant intergranularconnectivity characteristic. The method of the present inventionrecognizes the need to balance these competing processes and provides aheat treatment which maximizes the desired intragranular electricalproperty, while minmizing the degradation of intergranular connectivity.The method includes heating a (Bi,Pb)SCCO-2223 oxide phase underconditions which modify the (Bi,Pb)SCCO-2223 phase to optimizeintragranular electrical properties and to localize any secondary phaseformed in post-processing heat treatments to regions of the oxidesuperconductor composite where it is benign to supercurrent flow,thereby optimizing intergranular connectivity.

In the case of sheathed BSCCO-2223 wires, the following issues need tobe addressed when seeking to enhance intragranular properties withoutcost to grain interconnectivity.

Processing conditions (T, P_(O2), t) must be sufficient to diffuseoxygen through the substantially dense ceramic filaments and themetallic sheath. This is mainly a kinetic effect and oxygenation can beaccomplished, for example in a silver-based alloy sheathed system, byuse of temperatures greater than 500° C., at times in excess of 1 hoursand oxygen partial pressures of greater than 0.01 atm.

Processing conditions must also be selected such that secondary phasematerial, when formed, occupies a position in the microstructure whichis benign to supercurrent flow in the wire. Secondary phases may arisein several possible situations. Oxygen stoichiometry change may lead toa change in cationic states within the oxide superconductor, resultingin material being exsolved (expelled) from the superconductor phase. Alikely candidate for exsolution is lead (Pb), which may undergo anoxidation valance change from Pb²⁺ to Pb⁴⁺ during exsolution.Alternatively, changes in the thermodynamic state may causedecomposition of the oxide superconducting phase. Some types of phasedecomposition may result in enhanced flux-pinning. For example, verysmall oxide secondary phases (10-5000 Å) within the superconductor oxidephase on the order of the coherence length of the superconductingelectron pairs can pin magnetic vortices. In either case, it isdesirable that secondary phases that do not create vortices pinningoccupy a position in the oxide superconductor composite where they aresubstantially benign to supercurrent flow.

Regardless of the driving force to the intragranular change in the oxidesuperconducting phase, a secondary phase is formed. In order to reap thebenefits of the intragranular phase modification of the oxidesuperconductor, the secondary phase desirably does not disrupt theintergranular connectivity of the composite.

Applicants have discovered that by careful control of the processingconditions by which the oxide superconductor phase modification occurs,formation of the secondary phase may be localized at high energy sites.Since supercurrent flow occurs preferably at low energy sites, graininterconnectivity is not disrupted. Localization of secondary phases athigh energy sites may be accomplished by balancing the energy ofdecomposition (of the oxide superconductor phase to the secondary phase)and the rate of entropy increase of a secondary phase at the variousmicrostructural sites.

Decomposition in a closed materials system, such as a sheathed hightemperature superconductor wire, has an associated energy. The magnitudeof this energy depends upon the specific phases and the microstructurebefore and after decomposition. In the present case, BSCCO-2223 has asmall thermodynamic state stability field, and is relatively difficultto form. As a result, there is a strong driving force for thedecomposing of BSCCO-2223 during the transition from conditions ofBSCCO-2223 formation to ambient conditions.

During "ramping" conditions (approach to formation conditions or returnto ambient conditions), the principles of irreversible thermodynamicscontrol microstructural evolution. This is in contrast toisothermodynamic state treatments in which the principles of equilibriumthermodynamics (minimization of the free energy of the system) control.A governing principle of irreversible thermodynamics is that the timerate of entropic increase is maximized. In the present case, oneattempts to simultaneously control equilibrium and irreversiblethermodynamic considerations in order to control microstructuralevolution.

With respect to the microstructure, every structure within the closedmaterials system has some associated free energy. For example, theenergy of a grain boundary increases as the number of broken chemicalbonds associated with it increases. Thus, a high-angle grain boundary,where there is a greater degree of PbO-PbO₂ "bond mismatch" betweenneighboring grains, is likely to have a higher free energy than a lowangle grain boundary. Other examples of high energy sites includes (a)high angle c-axis tilt boundaries, (b) pores, (c) interfaces betweensuperconducting and secondary phases, and (d) surface boundaries(boundaries terminating perpendicular to the c-axis) for thesuperconducting phase. Examples of low energy sites within thesuperconducting composite include (a) within the superconducting grains,(b) c-axis twist boundaries (tilt=0), (c) c-axis boundary with thesilver phase, (d) coincident site lattice boundaries and (e) twinboundaries.

Because the energy associated with high energy sites is high, there is astrong driving force to "grow" the decomposition products at that pointwhich decreases overall free energy of the system. Thus, if time rate ofentropic increase associated with the decomposition of the oxidesuperconductor phase is small, then the decomposition products will growat high energy sites. However, if the time rate of entropic increase ishigh then decomposition products will form at low and high energy sites.Note that mass transfer to high energy sites is more substantial thanmass transfer to low and high energy sites. A practical means tomaintain a low time rate of entropic increase (so that high energy sitesare favored) is to hold the article at a thermodynamic state that isclose to, but outside that of, the desired superconductor. If theprocess state is far from the thermodynamic state of the desiredsuperconductor, irreversible thermodynamics governs microstructuralevolution.

It follows then that, in order to obtain a silver sheathed BSCCO-2223wire with enhanced critical current and critical current retention, oneprocesses the wire under conditions that are very close to the phaseboundary between BSCCO-2223 and the decomposition phase, therebyminimizing the force driving secondary phase formation at low and highenergy sites as indicated in FIG. 13. In prior art compositions,secondary phase growth occurs without selectivity at both low and highenergy sites. In such instances, the rate of entropic increase is highand secondary phase growth is indiscriminate.

According to the invention, the BSCCO-2223 wire is heat treated in aprocessing space in which the decomposition products form at high energysites. The decomposition reaction of interest is one which achieves thedesired enhancement of intragranular properties. This processing spacebalances the irreversible and equilibrium thermodynamics and has theadditional benefit of minimizing the absolute magnitude ofdecomposition. Thus, the heat treatment of the invention simultaneouslyminimizes the formation of the secondary phase and its detrimentaleffect to the electrical properties of the composite.

In one embodiment of the invention, modifications of the lead content inthe oxide superconducting phase achieve the desired results. Underoxidizing conditions, lead is exsolved (expelled) from the oxidesuperconducting phase, presumably undergoing a change in valence statefrom 2+ to 4+. The exsolved lead forms a secondary phase which has ahigh lead content. The formation of the lead-rich non-superconductingphase is associated with the reduction of lead in the (Bi,Pb)SCCO-2223oxide superconductor phase. Lead loss in (Bi,Pb)SCCO-2223 may be in therange of about 5 wt % to about 50 wt % lead. Preferably lead loss is inthe range of about 15 wt % about 25 wt %. Lead loss is reported as apercentage of the amount of lead originally present in the(Bi,Pb)SCCO-2223. It is contemplated that other metal capable of +2 to+4 (or +1 to +3) valance state changes that are soluble in the oxidesuperconductor phase may be used according to the invention. Suitablecations include, but are in to way limited to, Pb, Tl, Sb, Te, Hg, Se,As and Sn.

The lead-rich secondary phase has a hexagonal crystal structure.Although the crystal symmetry does not change, the chemical compositionvaries with the temperature of formation. For example, a lead-richsecondary phase formed at 724° C. has an elemental composition,Bi:Pb:Sr:Ca:Cu, of 1:1:2:2:1, whereas a lead-rich phase formed at 784°C. has an elemental composition of 1:2:2:3:1. Both hexagonal lead-richphases have the same X-ray diffraction pattern, which is shown in FIG.1a. The diffraction pattern corresponds to that catalogues in the JCPDSfiles as 44-0053. FIG. 1b is an X-ray diffraction pattern of a(Bi,Pb)SCCO-2223 composite of the invention including the lead richsecondary phase. Peaks marked with an asterisk are attributable to thesecondary lead-rich phase. The remaining peaks are attributable to theBSCCO-2223 oxide superconductor. Flukiger et al. have previouslyobserved this phase and the interested reader is directed to Physica C235-240:505-506 (1994) for further information, herein incorporated byreference.

The marked improvement of critical current retention in the oxidesuperconductor wires of the present invention correlates to thereduction of lead content in the oxide superconducting phase. While notbeing bound to any particular theory of operation, it is hypothesizedthat the change in oxygen activity of lead (Pb) leads to a decrease inthe lead content in the oxide superconductor. The altered stoichiometrymay introduce oxygen defect sites which are effective flux pinning sitesand/or change the intrinsic coupling. Flux pinning sites are known toimprove critical current performance in a magnetic field.

The heat treatment of the invention should satisfy both the kinetic andthermodynamic criteria set forth above, that is, the heat treatmentshould support material transport throughout the composite and shouldsupport oxidations of the divalent metal dopant in the BSCCO-2223 phasewhile remaining close to the stability line of the decomposition phase.A reasonable guideline in determining the appropriate processingconditions is to process under conditions of the unitary oxide stabilityphase, e.g.; PbO--Pb₃ O₄ --PbO₂ in the context of the appropriatemulticomponent oxide phase. FIG. 2 is a plot of log P_(O2) vs. 1000/K(K⁻¹) on which a calculated stability curve 20 for the lead-rich phaseis shown. The region above the plot represents conditions which areoxidizing for Pb²⁺, resulting in formation of the lead-rich phase. Thetemperature range is bounded one the low side by kinetic considerationsand on the high side by concerns for indiscriminate mass transfer. Inone embodiment of the invention, the heat treatment is conducted in aspace 22 at pressures above the lead-rich phase reaction curve 20 over atemperature on the range of about 500° C. to 800° C. In a preferredembodiment, heat treatment is conducted in a space 24 above thelead-rich phase reaction curve 20 over a temperature in the range ofabout 790° C. to 630° C. and in a most preferred embodiment, the heattreatment is conducted in a space 26 at pressures above the lead-richphase reaction curve 20 over a temperature on the range of about 650° C.to about 750° C. Upper limit on oxygen pressure is about 100 atm.

To summarize, heat treatment used according to the present invention maybe in the range of 800° C. to about 500° C. at an oxygen content of 0.03to 100 atm. Preferably the heat treatment is conducted at a temperaturein the range of about 790° C. to about 630° C. and most preferably at atemperature in the range of about 650° C. to about 750° C. The oxygenpressure is preferably in the range of 0.075 atm to 1.0 atm O₂, suchthat the pressure is above the reaction curve 20 at all times.

The heat treatment may be carried out in a variety of ways, as indicatedin FIG. 3. The heat treatment may be a single "bake" at a singletemperature (FIG. 3a) or it may be a series of shorter "bakes" asprogressively lower temperatures (FIG. 3b). Alternatively, the heattreatment may be accomplished by a very slow ramp (cool down) throughthe temperature range of interest, so that the total dwell time in theeffective temperature is achieved (FIG. 3c). Curves 30 and 32 bound theeffective temperature range for the heat treatment. Preferred dwell timeis greater than 20 hour and preferably greater than 30 hours.

The composite is desirably subjected to preliminary treatment in orderto provide good intergranular connectivity prior to the post-formationheat treatment of the present invention. Good grain interconnectivity isaccomplished by proper alignment of the oxide grains and substantiallycomplete conversion of the oxide precursor materials into the BSCCO-2223oxide superconductor. Conventional methods are available in the priorart to accomplish this. Suitable methods are described hereinbelow.

Any conventional method may be used to prepare the (Bi,Pb)SCCO-2223phase used in the present invention. A preferred method for preparationof a (Bi,Pb)SCCO-2223 oxide superconductor phase is a multistep heattreatment. Heat treatments at different points in the process play adifferent role in the manufacture of the (Bi,Pb)SCCO-2223 composite.After thermomechanical processing of the precursor oxide(typically,(Bi,Pb)SCCO-2212) into a wire of desired orientation anddimension (see below), a multistep heat treatment is carried out toconvert the precursor oxide into (Bi,Pb)SCCO-2223. The first step of theheat treatment is conducted at a relatively high temperature underconditions sufficient to form a liquid phase to partially melt the oxidephase which heals cracks induced in previous deformation processing andconverts (Bi,Pb)SCCO-2212 into (Bi,Pb)SCCO-2223. The second step of theheat treatment at a slightly lower temperature converts any liquid atthe (Bi,Pb)SCCO-2223 grain boundaries formed in the previous heattreatment into (Bi,Pb)SCCO-2223. An optional third step of the heattreatment at an even lower temperature "cleans" the (Bi,Pb)SCCO-2223grain boundaries (reacts away undesirable phase impurities) to obtaingood intergranular connectivity and completes conversion of theprecursor to (Bi,Pb)SCCO-2223. A typical heat profile is shown in FIG.4, where T₁ =850°-800° C., and preferably 830°-825° C. (40 h, 0.075 atmO₂), T₂ =815°-780° C., and preferably 813°-805° C. (40 h, 0.075 atm O₂)and T₃ =790°-780° C., and preferably 787° C. (30 h, 0.075 atm, O₂). Theinterested reader is directed to co-pending application U.S. Ser. No.08/041,822 filed Apr. 1, 1993, now U.S. Pat. No. 5,635,456 the contentsof which are herein incorporated in its entirety by reference.

The (Bi,Pb)SCCO-2223 phase is substantially single phase 2223; however,100% conversion may not always be obtained. Small amounts of startingmaterials and/or other non-superconducting phases may be present. Theyshould not be present at levels greater than 10 vol %, and preferablyless than 5 vol %.

The composite is desirably subjected to preliminary thermomechanicaltreatment in order to orient or texture the precursor (Bi,Pb)SCCO-2212oxide grains before their conversion to (Bi,Pb)SCCO-2223. Knownprocessing methods for texturing superconducting oxide compositesinclude combination of heat treatments and deformation processing(thermomechanical processing). BSCCO-2212 superconducting oxide grainscan be oriented along the direction of an applied strain, a phenomenonknown as deformation-induced texturing (DIT). Deformation techniques,such as pressing and rolling, have been used to induce grain alignmentof the oxide superconductor c-axis perpendicular to the plane ordirection of elongation. Heat treatment under conditions which at leastpartially melt and regrow the BSCCO-2212 superconducting phase also maypromote texturing by enhancing the anisotropic growth of thesuperconducting grains, a phenomenon known as reaction-induced texturing(RIT).

Typically, density and degree of texture are developed in the compositeby repeated steps of deformation (to impart deformation-inducedtexturing) and sintering (to impart reaction-induced texturing). Thesteps of deforming and sintering may be carried out several times. Theprocess may be designated by the term "nDS", in which "D" refers to thedeformation step, "S" refers to the sintering or heating step and "n"refers to the number of times the repetitive process of deformation andsintering are carried out. Typical prior art processes are 2DS or 3DSprocesses. See, Sandhage et al. (JOM 21 (March, 1991)), hereinincorporated by reference. A 1DS process is described in co-pendingapplication, U.S. Ser. No. 08/468,089 filed Jun. 6, 1995 and entitled"Simplified Deformation-Sintering Process for Oxide SuperconductingArticles", which is herein incorporated by reference. The nDS processmay be used to orient the precursor oxide phase before its conversioninto the (Bi,Pb)SCCO-2223 oxide superconductor wire.

The oxide superconductors which make up the oxide superconductorarticles of the present invention are brittle and typically would notsurvive a mechanical deformation process, such as rolling or pressing.For this reason, the oxide superconductors of the present invention aretypically processed as a composite material including a malleable matrixmaterial. The malleable material is preferably a noble metal which isinert to oxidation and chemical reaction under conditions used in theformation and post-formation processing of the composite. Suitable nobelmetals include palladium, platinum, gold, silver and mixtures thereof.In particular, silver is preferred as the matrix material because of itscost, nobility and malleability. The oxide superconductor composite maybe processed in any shape, however, the form of wires, tapes, rings orcoils are particularly preferred. The oxide superconductor may beencased in a silver sheath, in a version of the powder-in-tubetechnology. The oxide superconductor can take the form of multiplefilaments embedded within a silver matrix. For further information onformation of superconducting tapes and wires, see Sandhage et al.

The advantages of the post-formation heat treatment of the presentinvention is demonstrated in the following examples, which are presentfor the purpose of illustration only and which are in no means intendedto be limiting of the invention. Note that J_(c) values are criticalcurrent density normalized to reflect the different superconductingcontent of the wires. J_(c) is the critical current carried by an oxidesuperconductor filament. In the instance of a multifilamentary oxidesuperconductor wire, J_(c) is a value obtainable by division of thetotal current of the oxide superconductor multifilamentary wire by theoxide superconductor cross-sectional area. J_(e) is the critical currentover the entire cross-sectional area of the multifilamentary wire, avalue obtainable by division of the total current by the cross-sectionalarea of the wire. Comparison of J_(e) among wires having differentfill-factors is not meaningful, however, wires having the same fillfactor may be readily compared.

A multifilamentary oxide superconductor tape (85 filament count) isprepared from (Bi,Pb)SCCO-2212 powders having the overall composition ofBi(1.74):Pb(0.34):Sr(1.9):Ca(2.0):Cu(3.03) as follows. Precursor powderswere prepared from the solid state reaction of freeze-dried precursor ofthe appropriate metal nitrates having the stated stoichiometry. Bi₂ O₃,CaCO₃, SrCO₃, Pb₃ O₄ and CuO powders could be equally used. Afterthoroughly mixing the powders in the appropriate ratio, a multisteptreatment (typically, 3-4 steps) of calcination (800° C. ± 10° C., for atotal of 15 h) and intermediate grinding was performed to homogenize thematerial and to generate the BSCCO-2212 oxide superconductor phase. Thepowders were packed into silver sheaths to form a billet. The billetswere drawn and narrowed with multiple die passes, with a final passdrawn through a hexagonally shaped die into silver/oxide superconductorhexagonal wires. Eighty-five (85) wires were bundled together and drawnthrough a round die to form a multifilamentary round wire.

The round multifilamentary tape is heated at 760° C. for 2 hours in0.001 atm O₂ and rolled to the desired thickness in a single draftprocess (from about 35.4 mil to about 6 mil). Heat treatment at 827° C.(0.075 atm O₂) for 40 h and at 808° C. (0.075 atm O2) for 40 h are usedto convert the (Bi,Pb)SCCO-2212 phase into (Bi,Pb)SCCO-2223.

The effect of the temperature, oxygen content and dwell time of thepost-formation heat treatment on superconducting properties,microstructure and composition were investigated. A four-point probe wasused to measure critical current, with a voltage criterion of 1 μV/cmfor the determination of J_(e).

The temperature of the post-formation heat treatment was systematicallyvaried while atmosphere and dwell time was held constant (0.075 atm O₂,30 h). FIG. 5 is a plot of J_(e) performance as a function oftemperature (500° C. to 800° C.). All measured J_(e) represented animprovement over pretreatment performance. Optimal J_(e) performance(ca. 11,600 A/cm²) was measured at a temperature in the range of700°-724° C. Improvements in J_(e) represent improvements in bothintragrain and intergrain characteristics of the oxide superconductor.

For a different set of samples, FIG. 6 is a plot of J_(e) as a functionof dwell time for oxide superconductor wires post-formation heat treatedat 724° C. in 0.075 atm (7.5%) O₂. Significant improvement in J_(e) withdwell time is observed, with diminishing incremental improvement asdwell time increases above 6 h.

The oxygen partial pressure of the post-formation heat treatment alsowas systematically varied while temperature and dwell time was heldconstant (724° C., 30 h). FIG. 7 is a plot of J_(e) as a function ofoxygen partial pressure (0.003-1.0 atm). Balance of gas is inert gas,such as nitrogen or argon. Optimal J_(e) performance (ca. 11,500 A/cm²)was measured at an oxygen partial pressure in the range of 0.075 atmoxygen. Interestingly, optimal T_(c) was obtained at lower oxygenpartial pressures and optimal J_(ret) was obtained at 1.0 atm oxygen.See, FIG. 7. This is a dramatic illustration of how optimization forparticular intergranular and intragranular properties occurs indifferent processing regimes.

In conclusion, it is apparent that temperature, oxygen partial pressureand dwell time may be varied to optimize the absolute J_(e) performanceof the (Bi,Pb)SCCO-2223 wire. A preferred post-formation heat treatmentfor optimal J_(e) is at a temperature in the range of about 700° C. to730° C.; at an oxygen partial pressure of about 0.075 atm O₂ ; and at adwell time of at least about 20 hr, and preferably at least about 30 h.Other preferred conditions are within the scope of the invention. Forexample, at higher oxygen pressures, the preferred temperature shoulddecrease and the dwell time is expected to increase.

The ability for the (Bi,Pb)SCCO-2223 wire to retain critical current inan applied magnetic field was also investigated. A (Bi,Pb)SCCO-2223 wiresubjected to the heat treatment of the invention demonstrates aretention of up to about 40% and preferably about 25% to about 35% ofcurrent carrying capacity at 0.1 T (77 K, ⊥ tape surface). Fieldstrengths of this magnitude are of interest because they are comparableto the applied field in certain applications. FIG. 8 is a bar graphillustrating J_(ret) for a variety of post-formation heat treatments.All samples retained at least 25% of initial critical current density.J_(ret) showed a maximum at heat treatments of 724° C./1 atm O₂ /30 h.These J_(ret) performances represent a significant improvement overperformances reported in the prior art.

FIGS. 9 and 10 are plots of J_(c) retention vs. field strength for(Bi,Pb)SCCO-2223 wires heated under different oxygen partial pressuresand temperatures, respectively, which demonstrate that critical currentflow remains in fields up to 0.5 T.

Note that samples processed for maximum J_(e) do not necessarily alsoexhibit optimal critical current retention. Table 1 shows the T_(c),J_(e), and J_(ret) values for samples held at constant pressure (0.075atm, 30 h) at varying temperatures (Ex. 1-5) and for samples held atconstant temperature (724° C., 30 h) at varying oxygen pressures (Ex.6-8). J_(e) values can only be compared within a samples series, as fillfactor changes. Note that optimal T_(c), optimal J_(c) and optimalJ_(ret) result at different processing conditions. This supports theearlier observation that factors maximizing the two properties need notbe identical.

                                      TABLE 1                                     __________________________________________________________________________    T.sub.c, J.sub.e, and J.sub.ret values for variously heat-treated             samples.                                                                      parameter       T.sub.onset                                                                      ΔT.sub.c                                                                   J.sub.e *                                               No.                                                                              constant                                                                              variable                                                                           (K)                                                                              (K)                                                                              (amp/cm.sup.2)                                                                     J.sub.ret (%)                                                                     comments                                       __________________________________________________________________________    1  0.075 atm O.sub.2,                                                                    500° C.                                                                     107                                                                              6.5                                                                                6500                                                                             26                                                    30 h                                                                       2  0.075 atm O.sub.2,                                                                    700° C.                                                                     108                                                                              4.0                                                                              10,400                                                                             29                                                    30 h                                                                       3  0.075 atm O.sub.2,                                                                    724° C.                                                                     108                                                                              3.3                                                                              11,600                                                                             32                                                    30 h                                                                       4  0.075 atm O.sub.2,                                                                    750° C.                                                                     109                                                                              4.0                                                                                9600                                                                             29  optimal                                           30 h                        T.sub.c.sup.+                                  5  0.075 atm O.sub.2,                                                                    800° C.                                                                     109                                                                              8.0                                                                                7500                                                                             30                                                    30 h                                                                       6  724° C., 30 h                                                                  0.03 atm                                                                           108                                                                              4.0                                                                                8000                                                                             25                                                 7  724° C., 30 h                                                                  0.075 atm                                                                          108                                                                              3.0                                                                              11,600                                                                             32  optimal J.sub.c                                8  724° C., 30 h                                                                  1.0 atm                                                                            105.5                                                                            5.0                                                                                5100                                                                             37  optimal J.sub.ret                              __________________________________________________________________________     *J.sub.e values may only be compared within the same sample series.           .sup.+ note that both sample nos. 4 and 5 have comparable T.sub.onset,        however, sample no. 4 has a smaller ΔT.                            

Formation of a new lead-rich non-superconducting phase is observedduring the post-formation heat treatment of the invention and the amountof this phase increases with dwell time. This phase was not observedduring (Bi,Pb)SCCO-2223 formation heat treatments. The appearance of thelead-rich secondary phase and the increased formation with increaseddwell time correlates well with the observed improvements in J_(c) andJ_(ret) in the post-formation heat treatment. FIG. 11 is a plot of therelative fraction of the lead-rich secondary phase in the final(Bi,Pb)SCCO-2223 wire as a function of oxygen partial pressure (724° C.,30 h). The relative fraction of the lead-rich phase increases withincreased oxygen partial pressure. This correlates well with conditionsproducing the maximum critical current retention. Note that no lead richsecondary phase appears to have formed at 0.003 atm oxygen, theprocessing condition which optimized T_(c). Thus, the appearance of thisphase positively affects the performance of the post-formation heattreatment samples. FIG. 12 is a plot of the relative fraction of thelead rich phase as a function of temperature. Curve 110 is for samplesat 0.075 atm O₂. Significant lead-rich secondary phase formation isobserved for temperatures in the range of 724°-775° C. Results from FIG.11 (at 724° C.) are included in this plot and suggest that at higherP_(O2), a greater temperature range may provide significant amounts ofthe lead-rich phase.

The effect of heat treatment on critical current density retention aboveand below the lead-rich phase stability line is demonstrated in FIG. 13.Curve 120 represents the J_(ret) for samples processed under conditionsabove the lead-rich phase stability line. Open circle data pointrepresent samples heat treated at 724° C. at 0.075 atm for 30 hours.Closed diamond data points represent samples heat treated at 724° C. at1.0 atm for 30 hours. Curve 122 represents the J_(ret) for samplesprocessed at 724° C. at 0.003 atm for 30 hours--conditions below thelead-rich phase stability line. Performance represented by curve 122 issignificantly compromised.

The lead-rich secondary phase formation and the effect of its formationon (Bi,Pb)SCCO-2223 were investigated by scanning electron microscopy(SEM) and energy dispersive spectrometry (EDS), which permitteddetermination of the elemental composition of both phases. The resultsare reported in Table 2. The relative stoichiometry of the(Bi,Pb)SCCO-2223 phase prior to the post-formation heat treatment isconsistent with a nominal 2:2:2:3 stoichiometry. However, after heattreatment, the level of lead in the superconducting phase has beenreduced significantly and a secondary phase rich in lead and poor incopper is formed. The relative fraction of the lead-rich secondary phaseincreases with dwell time and appears decorating the perimeter of theBSCCO-2223 grains. Further, the lead-rich phase appear to concentrate atthe BSCCO-2223 high-energy sites.

                                      TABLE 2                                     __________________________________________________________________________    (Bi,Pb)SCCO-2223 and lead-rich phase compositions (at. %).                           last heat                                                              compound                                                                             treatment                                                                          Bi   Pb   Sr   Ca   Cu                                            __________________________________________________________________________    (Bi,Pb)SCCO-                                                                         808° C.                                                                     19.8 ± 0.4                                                                       4.9 ± 0.1                                                                      20.9 ± 0.5                                                                      23.6 ± 0.3                                                                      30.8 ± 0.6                                 2223   (40 h)                                                                 post-formation                                                                       724° C.                                                                     21.1 ± 0.5                                                                       3.8 ± 0.4                                                                      22.5 ± 0.6                                                                      22.9 ± 0.4                                                                      29.7 ± 1.4                                 heat treated                                                                         (30 h)                                                                 (Bi,Pb)SCCO-                                                                  2223                                                                          lead-rich phase                                                                      724° C.                                                                     14.2 ± 1.5                                                                      17.7 ± 3.3                                                                        25 ± 0.9                                                                      27.1 ± 1.3                                                                        16 ± 3.6                                        (30 h)                                                                 __________________________________________________________________________

Although not intending to be limited to a single interpretation, onepossible explanation for the observed appearance of the lead-rich phaseunder conditions which also improve critical current retention is thatthe starting composition is overdoped with lead and that the extra leaddecomposes into the lead-rich secondary phase. An alternativeexplanation is that the lead-rich phase is the product of oxygen contentmodification in the (Bi,Pb)SCCO-2223 lattice. In other words, oxygendefects are introduced into the 2223 lattice for high performance andthe doped oxygen defects change the valance of the Pb in the 2223lattice and consequently the lead-rich phase is forced to decompose outfrom the 2223 phase. Further, the increased flux pinning is derived fromthe introduction of oxygen defect Therefore, the post-formation heattreatment results in both oxygen defect formation and lead-rich phaseformation, which influences both the intergranular and intragranularproperties of the superconductor.

Other embodiments of the invention will be apparent to the skilled inthe art from a consideration of this specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A method of processing a (Bi,Pb)SCCO-2223 oxidesuperconductor composite after oxide superconductor phase formation,comprising:providing an oxide superconductor composite wire or tapecomprising (Bi,Pb)SCCO-2223; heating the oxide superconductor compositewire or tape under conditions selected to reduce the lead content of the(Bi,Pb)SCCO-2223 oxide superconductor by about 5 percent to about 50percent by weight and to localize the exsolved lead in a secondary phaseat high energy sites of the composite, whereby the oxide superconductingcomposite wire or tape is characterized by retention or improvement ofelectrical transport carrying properties, wherein said conditionscomprise heating said composite wire or tape above 500° C. for at least6 hours.
 2. A method of processing a (Bi,Pb)SCCO-2223 oxidesuperconductor composite after oxide superconductor phase formation,comprising:providing an oxide superconductor composite wire or tapecomprising (Bi,Pb)SCCO-2223; heating the oxide superconductor compositewire or tape under oxidizing conditions, said conditions sufficient tooxidize a portion of Pb²⁺ present in (Bi,Pb)SCCO-2223 into Pb⁴⁺ and tolocalize the Pb⁴⁺ in a secondary phase at high energy sites of thecomposite, whereby the oxide superconducting composite wire or tape ischaracterized by retention or improvement of electrical transportcarrying properties, wherein said oxidizing conditions comprise heatingsaid composite wire or tape above 500° C. for at least 6 hours.
 3. Themethod of claim 1 or 2, wherein the high energy site comprises one ormore sites selected from the group consisting of high angle c-axis tiltboundaries, pores, interfaces between the superconducting and secondaryphases and edge boundaries for the superconducting phase.
 4. he methodof claim 1 or 2, wherein the heating step is carried out under oxidizingconditions.
 5. The method of claim 1 or 2, wherein the heat treatment iseffective to provide a composite which exhibits a critical currentretention at 0.1 T (77 K, ⊥ ab plane) in the range of about 15% to about50%.
 6. The method of claim 1 or 2, wherein the heat treatment iseffective to provide a composite which exhibits a critical currentretention at 0.1 T (77 K, ⊥ ab plane) in the range of about 20% to about40%.
 7. The method of claim 1 or 2, wherein the (Bi,Pb)SCCO-2223 isprocessed to impart intergranular contact of the oxide superconductorgrains before heat treatment of invention.
 8. The method of claim 1 or2, wherein the heat treatment comprises:heating the composite at atemperature in the range of about 500° C. to about 800° C. at an oxygenpressure of about 0.03 atm to 100 atm O₂ for a time sufficient toprovide a critical current retention at 0.1 T of at least 15% (77 K, ⊥ab plane).
 9. The method of claim 8, wherein the temperature is in therange of 630° C. to 790° C. at an oxygen pressure of about 0.03 atm to100 atm O₂.
 10. The method of claim 8, wherein the temperature is in therange of 650° C. to 750° C. at an oxygen pressure of about 0.075 atm to1.0 atm O₂.
 11. The method of claim 1 or 2, wherein the lead-richsecondary phase comprises a hexagonal crystal structure characterized byan X-ray diffraction pattern comprising the following peaks (2θ(relativeintensity)): 17.9(45), 32.3(100), 31.5(62), 44.8(42), and 55.5(45). 12.The method of claim 1 or 2, wherein the (Bi,Pb)SCCO-2223 comprises leadin an amount in the range 3 wt % to about 8 wt % before heat treatment.13. The method of claim 1 or 2, wherein the (Bi,Pb)SCCO-2223 compriseslead in an amount in the range 4 wt % to about 6 wt % before heattreatment.
 14. The method of claim 1 or 2, wherein the (Bi,Pb)SCCO-2223comprises about 6.5 wt % lead.
 15. The method of claim 1 or 2, whereinthe heat treatment is carried out under conditions to reduce the leadcontent of (Bi,Pb)SCCO-2223 in an amount in the range of about 15 wt %to about 25 wt %.
 16. The method of claim 1 or 2, wherein the heattreatment comprises:heating the oxide superconductor under conditionswhich are oxidizing to Pb⁺² relative to a lead-rich phase stabilitycurve.
 17. The method of claim 1 or 2, wherein a (Bi,Pb)SCCO-2223 isobtained by heating in the range of 800° C. to 850° C. for a first dwelltime and heating in the range of 780° C. to 815° C. for a second dwelltime under an oxygen partial pressure in the range of 0.01 to 1.0 atm.18. The method of claim 1 or 2, wherein a (Bi,Pb)SCCO-2223 is obtainedby heating in the range of 825° C. to 830° C. for a first dwell time andheating in the range of 805° C. to 813° C. for a second dwell time underan oxygen partial pressure in the range of 0.01 to 1.0 atm.
 19. Themethod of claim 17, further comprising heating in the range of 780° C.to 790° C. for a third dwell time under an oxygen partial pressure inthe range of 0.01 to 1.0 atm.
 20. The method of claim 1, 2 wherein thecomposite is in the form of a silver sheathed wire.
 21. The method ofclaim 20, wherein the composite is a multifilamentary silver sheathedwire.
 22. The method of claim 1 or 2, wherein the lead-rich secondaryphase is formed in a relative fraction in the range of about 0.002 to0.5.
 23. The method of claim 1 or 2, wherein said composite (Bi,Pb)SCCO2223 oxide superconductor further comprises a noble metal.
 24. A methodof preparing a (Bi,Pb)SCCO-2223 oxide superconductor composite,comprising:providing an oxide superconducting composite wire or tapecomprising a (Bi,Pb)SCCO-2223 superconducting phase; and modifying thelead content of the (Bi,Pb)SCCO-2223 superconducting phase duringprocessing of a (Bi,Pb)SCCO-2223 oxide superconductor composite, suchthat the lead content of the (Bi,Pb)SCCO-2223 superconducting phase isin the range of 3% to 8% during formation of the (Bi,Pb)SCCO-2223 phaseand such that the lead content of the (Bi,Pb)SCCO-2223 superconductingphase is reduced up to 25% during post formation processing of the oxidesuperconductor phase, whereby the oxide superconducting composite wireor tape is characterized by retention or improvement of electricaltransport carrying properties, wherein modifying the lead contentcomprises heating said composite wire or tape above 500° C. for at least6 hours.